Direct-Current d-c motors which have an armature winding, energized in alternate directions from a direct current source under control of semiconductor switches, typically transistors, have previously been proposed. Switching-over of the polarity of the d-c supply source to the coil, to provide a pulsed field is effected under control of a rotor position sensor, which may be magnetically responsive to the magnet of the rotor itself, for example a Hall sensor, usually coupled to an integrated circuit amplifier to form a Hall-IC combination. The sensor controls energization of the armature winding via a controlled bridge circuit. This may be a full-wave or half-wave bridge.
The referenced U.S. Pat. No. 3,898,544, Tanikoshi, illustrates a two-wire or two-strand, four-pulse motor in which the two armature strands or wires are each connected and driven by a full-wave bridge circuit. Each bridge circuit is selectively energized by controlling transistors therein, the transistors, in turn, being controlled from two output signals of a Hall generator. The output signals from the Hall generator have an aproximately sinusoidal wave shape, that is, they rise only slowly, so that they will reach the switching voltage of the associated transistor only some time after having been subjected to the magnetic field from the rotor. At the commutation time instant, thus, a gap between current flow will result since the transistors will not switch instantaneously as the polarity of the rotor, to which the Hall generator is exposed, changes. The current gap, that is, the interruption of current flow, must have a certain time of sufficient length to prevent short circuits through the transistors with reliability and assurance. In order to have a sufficiently long current gap, transistors must be used which have similar threshold voltages, the Hall gnerators must have similar output signal characteristics, and the rotor must be magnetized, at least in the portion controlling the Hall generator, with approximately sinusoidal magnetization. All these conditions are cumulative and require expensive matching of components, and yet cannot be achieved at all times.